The lead agency in yesterday's Boston Marathon explosions is the FBI. Federal investigators say this morning they are doing all they can to get answers, but there is still much they do not know.

INSKEEP: Here's some of what we do know. Three people were killed, and the number of injured has now topped 150. That's according to Massachusetts Gov. Deval Patrick. The blasts occurred just before 3 in the afternoon yesterday, hours after the elite runners finished but while thousands were still on the course. NPR's Tovia Smith reports.

TOVIA SMITH, BYLINE: There are often tears at the finish line of the Boston Marathon, but nothing like yesterday.

JERRY FALGOUST: Ooh, there's my wife. Hey, honey.

CHERIE FALGOUST: (Crying) Hi, baby. Oh, baby...

SMITH: It was an emotional reunion for runner Jerry Falgoust and his wife, Cherie, who'd spent three hours looking for each other. At the time of the blast, he was still running - just about a half-mile from the finish line. And she was in the grandstands, rolling her video camera, waiting for him to cross.

CHERIE FALGOUST: Right after the first - just loud, boom! And then everybody ducking, something hit my head; it was probably just a small piece of plastic. And then it sounded almost like an echo of one. It was just very scary.

SMITH: Runners who were sprinting - or hobbling - toward the finish line began running the other way. Some were literally knocked to the ground by the blast. Buildings shook; windows shattered; and grandstands and steel barricades collapsed, crushing spectators on the sidewalk.

Stunned police even drew their guns, as everyone tried to figure out what had happened.

JAMES BRENNAN: Once the second one went off, I just said, you know, this is a bomb.

SMITH: James Brennan was working at a Boylston Street store just beside the explosion.

BRENNAN: It was just really scary to see people dropping to the floor like that. And there was just blood everywhere and there was, you know, people being carried in, in stretchers. People took the spools of yarn that we had to actually wrap up, you know, peoples' wounds, to try to stop the bleeding.

SMITH: Victims were rushed to area hospitals, and were also treated on the scene at medical tents that had been set up to deal with runners who were dehydrated or twisted their ankles.Instead, nurses like Edie Cardwell ended up triaging injuries they never could have imagined.

EDIE CARDWELL: I saw one guy being wheeled by me that lost two of - his legs. Just - numerous lacerations, just - chaos.

PRESIDENT BARACK OBAMA: We will find out who did this, and we will hold them accountable.

SMITH: While White House officials were calling the explosion terrorism, President Obama was more circumspect.

OBAMA: We still do not know who did this, or why, and people shouldn't jump to conclusions before we have all the facts. But make no mistake - we will get to the bottom of this.

SMITH: The FBI is now leading the investigation through the Joint Terrorism Task Force. Richard DesLauriers is the special agent in charge.

RICHARD DESLAURIERS: I encourage everyone to have a heightened state of vigilance here, in the Boston area, as we move forward. I urge anybody who has any information pertaining to this crime to call a tip line that we have set up.

SMITH: Authorities say they had no specific warning or threat of the attack; and two sweeps of the route turned up no problems, the latest around an hour before the first runners finished. But as they noted, it was an open course; and people could easily come and go.

Meantime, a huge swath around the blast - in the heart of Boston - remains cordoned off, and may continue to be treated as a crime scene, officials say, for days. Contrary to what they said yesterday, officials this morning said they have found no other devices besides the two that exploded. Still, Police Commissioner Ed Davis is urging caution.

ED DAVIS: We want you to live your life. We want you to be vigilant. There's no reason to not come into the city. But we do have a threat.

SMITH: Authorities say help from the public is critical to the investigation, and they're asking people to send in any photos or footage they have that may be helpful.

DAVIS: It's our intention to go through every frame of every video that we have, to determine exactly who was in the area. This is probably one of the most well-photographed areas in the country yesterday.

SMITH: As Boston remains on high alert, with extra police and spot checks in subways, for example, other cities - from Washington, D.C., to New York - have also tightened security. As Gov. Deval Patrick put it, Boston will be open for business, but it will not be business as usual.

DAVID WRAY: It just kind of makes you appreciate things a little bit more. (Starts crying)

SMITH: Runner David Wray, from Oklahoma, had already crossed the finish line in Boston when he saw the blast from his hotel room. Trish Farr, from Australia, also experienced the blast, shortly after she finished running. It may be that these runners, and the race, will never be the same.

TRISH FARR: It's the most prestigious marathon in the world. It's on everyone's bucket list; it's the one race that everyone really wants to do. But it's really frightening, and it's just tarnished what's a beautiful event.

SMITH: Yesterday's marathon began with 26 seconds of silence and special signs, all meant to commemorate those killed in the Sandy Hook Elementary School shooting in Connecticut. Next year, the Boston Marathon will have its own victims to remember.